Electron-emitting surfaces and methods of making them



Nov. 24, 1959 A. H. SOMMER 2,914,690

ELEcTRoN-EMITTING suRFAcEs AND METHODS oF MAKING THEM Filed Dec. 5, 19557001./ .225k 3T: l, ,2. .L

2.3i 42 'I f INVENTOR. j 4LP/F5@ H. S04/MM Fg' .BY

WMZ@ i A State Patentor Y photocathode iilm.

ELcTRoN-EMITTlNG SURFACES AND METHODS or MAKING THEM Alfred `HermannSommer, Princeton, NJ., assignor to Radio Corporation of America, acorporation of Delaware y i Application December 5, 195'5, VSerial No.551,028

9 Claims. (Cl. 313-65) This invention relates to electron dischargedevices having -iilms and to the art of making them, and, particularly,to electron-emitting iilms for use Yin phototubes, photomultipliertubes, camera tubes for television and the like.

Electron-emitting films emit electrons when bombarded by visible orinvisible radiation, charged particles, electrons or the like. Suchfilms may be made by properly depositing and reacting aplurality ofchemical substances on a support member of glass or metal or the like.One type of electron-emissive film emits electrons when bombarded byvisible or invisible radiation and is known as a photocathode.

- In general, a good photocathode film has high .and uniy formsensitivity, that is, the iilm emits the same large posite lsurface.Thus, the requirement yis imposed that the lm1be thin enough to bepenetrated and excited by the incident light. in order to replaceemitted electrons, the cathode iilm is electrically connected to a ttsource of electrons and another requirement is that the cathode iilm besufficiently thick to have the requisite conductivity to achieveelectron replacement. An optimum photocathode film Vis of uniformthickness so that electron emission and electron replacement areachieved uniformly overthe entire area thereof. However, present methodsof preparation do not provide such uniform `In the method of making aphotocathode in accordance with this invention, a suitable support plateof glass, or the like, isiirst provided with `a thin film of potassiumand then an electron-ernitting surface, such as a photo- Vcathodesurface, is formed thereon. The electron-emitting surface may be analkali-activated antimony system or the like. The thin potassium filmpromotes the formation of a uniform and sensitive-photocathode which viscomparatively easily reproduced. In `the drawing,

Fig, 3 is a sectional view along therline 3-3 in Fig. 2; and,

Fig. 4 is an enlarged' sectional view of a portion of the face plate andphotocathode in the tube shown in Fig. l.

yFor purposes `of illustration, the following description relates .to atelevision camera pickup tube which utilizes an alkali-antimonyphotoemissive surface or photocathode made according to the principlesof the invention. Referring to Fig. l, a television camera Itube 1,0,for example, ofthe image orthicon type, includes an `envelope having aface plate y14 on the inner surface of which 'a photoemissive surface orphotocathode 16 is provided 2,914,690 Patented Nov..`24, 195,9

ICC

according to the principles of the invention. voltages are shown inFigure 1 for purposes of illustrating the operation of the tube.A Thesevoltage values are not limiting. In operation of the tube of Fig. 1, ascene which is to be televised is optically focused upon the face plate14 and on the photocathode 16. Photo'electrons are released from thephotocathode'with a pattern of distribution corresponding to the`pattern of light coming from the scene to be televised. Thephotoelectrons are focused upon a thin glass target 18 by anelectromagnetic coil 20 which surrounds the envelope 12 and provides auniform magnetic field coaxial therewith. 'I'he photoelectrons are alsoaccelerated toward the glass target by ring electrodes 22 and 23 so thatthey strike the glass target 18 with sufficient energy to producesecondary emission therefrom. A ne mesh screen 24 is positioned closelyadjacent to the surface of the target 20 and collects the secondaryelectrons. The-loss of electrons from the surface of the glass targetleaves a positive charge pattern on the glass target surfacecorresponding in distribution in intensity to the optical scene focusedon the photocathode 16. Since the glass target is very thin, acorresponding pattern of potentials is. present on the surface away fromthe photocathode.

The last-mentioned surface of the glass target is scanned by an electronbeam formed'by an electron gun 26 located at the oppositeL end of thetube from the photocathode. The electron beam is aligned by oppositelydisposed coils 25 and 25', and is focused on the target 18 by the coil20. The beam is scanned over the surface of the Vtarget by deflectionfields produced by coils formed into a yoke 27, and is well known in theart. As the electrons of the beam approach the target 20 they aredecelera'ted to substantially zero velocity by means of a deceleratingring electrode 28. Some ofthe electrons from the cathode ray beam aredeposited on the positive areasl of the target surface in an amount toneutralize the positive charges on the opposite surface of the target.These deposited .electrons drive each positive target area to guncathode potential so that the remainder of the beam electrons arereflected back along the `tube envelope toward the electron gun 26 bythe neutralized target area. The reflected electrons are collected andamplified in a multiplier section `30 to form video signals in theoutput circuit of the tube (not shown).

by means of a strip of silver 32 (Fig. 2) painted on the inner wall ofthe envelope` 12 in contact with the photo- Y cathode 16. A springcontact lead 34 contacts the silver stripand is connected, as bywelding, to a pin 36 which extends `through the wall of the envelope.and to which electrical contact may ,be made. Similarly, theaccelerating electrodes 22 and 23 are welded to wire leads 37 and 38,respectively, which extend through the wall of the envelope.

A heater filament 40 of tungsten or the like, for ,use in evaporatingmaterial during the making of the photocathode 16, is disposed betweenthe mesh screen 24 and the electrode 23 as shown in Fig. 3. One end ofthe filament is secured to a lead 42 which passes insulatingly' extendsthrough the wall'of the envelope. The `other end of the filament 40 issecured to the electrode 23 by a lead 44. Thus, electrical connection tothe filament is provided through the external pins 38 .and 43. Three'other chemical carriers 45, 46 and 47 (Fig. 3), each vin the form ofdifferent elongated metal channels, as shown, or in any other convenientform, are also provided within the envelope 12. In one convenientarrangement, one end of each carrier 45,46 and 47 is secured to the,outer surface of accelerating electrode 23 and the other end-of eachcarrier is secured to wire leads 48, 49 and'50, re-

3 v v spectively,' .which extend through the Wall of the envelope.'Thus, external electrical connections are provided for passing currentthrough each of the chemical carriers. 45, 46 and 47. n

Inf assembling' the tube,the materials to be evaporated are prepared aspellets, powders or the like and are secured to or deposited in theirvariousV carriers or supportv members. Specilically, a plurality ofpellets or beads 52 comprising high grade commercial antimony with onlytraces `of iron, sulfur, arsenic and lead permitted are secured, latspaced intervals, on the filament 40. A mixture of one'part by weight ofpotassium chromate, one part by weight of` aluminum, Aand 8 parts byweight oftungsten is placed in the carrier 45. A mixture ofone part -byWeight lof sodium chromate, one part by Weight of aluminum and 8 partsby weight of tungsten isfplaced the carrier 46. A mixture of one part by'weight of cesium chromate `and 3 parts by weight of siliconY is placedin the carrier 47. The percentages of the Various components `of thelatter three mixtures disposed in the carriers 45, 46 yand 47 are notcritical and may be varied within Wide ranges, as desired.

After the various tube components are assembled in the envelope 12, thetube is mounted on an exhaust pump by means of an exhaust tubulation(not shown) and then baked yat a temperature in the range of 375 to 400C. for about one hour to remove occluded gases from the envelope andfrom the metal parts.

` The photocathode iilm 16 is prepared in the following manner: First,the tube is heated, for example in an oven, to a temperature in therange of 140 C. to 170 C. With the tube thus heated, a thin lm 54 (Fig.4) of potassium is evaporated onto the inner surface of the face plate14 of the envelope 12. This evaporation is effected by passing currentfrom a suitable power source (not' shown) through the leads 38 and 43and heating the carrier 45 and thereby evaporating potassium metalreleased by the reaction of the potassium chromate, aluminum andtungsten. As the potassium is deposited on the face plate thephotoelectron emission therefrom is measured and when the emissionreaches a peak value, the evaporation of potassium is discontinued.

This electron emission from the potassium ilm may be measured byconnecting the photocathode by its lead 36 andthe electrode 23 by itslead 38 in series with an ammeterk56 and then directing light from asource 58 ontothe face plate. The tube is kept at an elevatedtemperature as described above to render the glass of the face plateconductive so that the foregoing measurement of the potassium film maybe made during the evaporation of the potassium. The potassium lm thusformed'is invisible and is believed toI -be in the form of a monatomiclayer. Next, a film 60 of antimony is deposited over the potassium lm 54on the face plate 14. This operation may be performed with the tube atroom temperature. In order'to evaporate the antimony from the pellets 52a Vheating current is passed through the leads 38 and 43 to heat thefilament 40 to the vaporization temperature for the antimony metal. Forreasons which are not fully understood, the presence of the potassiumiilm on the face plate promotes the formation of a thinner, more uniformantimony film than would be formed on the glass face plate alone. Thedeposition of the antimony may be continued until the light transmissionfrom the source 58 through the face plate is in the range of 50 percentto 95 percent of the light passed prior to the formation of the film.The light transmission through the face platemay be measured by means ofa photocell in the manner disclosed in U.S. Patent No. 2,676,282 of I.I. Polkosky. The light transmission prior to the deposition of the tilmmay be arbitrarily assumed to be 100. To form' one type of photocathode,the antimony iilm '60 is then activated with three alkali metals,potassium,

Y 4 l Y sodium, and cesium. First, the tube is again heated to atemperature in the range 'of 140 C. to 170 C. in order to control theamount of potassium deposited on the lm 60. Potassium is then evaporatedonto the antimony tilm from the material remaining in the carrier 45 bypassing heating cun'ent therethrough by Way of the pins 38 and 48. Theevaporation of the potassium is continued until the photo-emissionreaches a peak value as indicated by ammeter 56. Evaporation ofpotassium is then discontinued and the temperature of the tube is raisedto about 220 C. and the excess potassium metal is pumped out.

The tube is then held at a temperature in the range of 180 C. to 220 C.Yand the antimony film is treated `with a second alkali metal, sodium,which is evaporated onto the coated face plate by heating land reactingthe material in the carrier 46. The requisite heating may be achieved bypassing current through the leads 38 and 49. The evaporation of sodiumVis also continued until peak photoemission is achieved and the excessis then pumped out of the tube.

Finally, the tube temperature is brought into the range of 130 C. to 160C. and cesium is evaporated onto the coated face plate from the carrier47 until electron emission from the photocathode reaches a peak valueand the excess cesium is pumped out. Thus, the photocathode 16 isessentially complete and the tube is cooledand processed further asrequired.

Under some circumstances, it may be desirable to evaporate additionalantimony onto the photocathode, as it is formed, after each activationwith an alkali metal'.

In the foregoing process, the order in which the sodium and potassiumare evaporated onto the antimony coated face plate may be reversed. Inaddition, the principles of the invention may also be employed withsecondary electron-emissive coatings and with other photocathodesystems, for example, antimony-potassiumrubidium,antimony-rubidium-potassium, antimony-potassium-sodium,antimony-sodium-potassium, antimony-potassium lithium or the like. Ineach case, the selected photocathode system is formed on a face platecoated with a iilm of potassium.

The method of the invention provides electron emitting surfaces whichexhibit sensitivities over their areas which do not vary by more thanabout 3 percent to 7 percent. For example, `one photocathode showedsensitivities varying Vover the whole surface in the range of l70 to 175microamperes per lumen of incident light. On the other hand,photocathodes made according to prior art practices have sensitivitiesover their areas which may vary up to percent. For example, one suchphotocathode had sensitivities in the range of 20 to 110 microamperesper lumen.

Thus, the method of the invention provides electronemitting surfaceswhich are reproducible and which have high and uniform sensitivity.

What is claimed is:

l. An electron-emissive electrode comprising a supporting base, a ilm ofpotassium on said base, a film of antimony on said potassium film, saidantimony lm including reaction products of antimony with a plurality ofalkali metals.

2. An electron-emissive electrode comprising a supporting base, a iilmof potassium on said base, a film of antimony on said potassium film,saidantimony film including reaction products of antimony with sodiumand potassium.

3. An electron-emissive electrode comprising a supporting base, a filmof potassium on said base, a lm of antimony on said potassium film, saidantimony tilm including reaction products of antimony with sodium,potassium and cesium.

4. An electron-emissive electrode comprising a supporting base, a filmof potassium on said base, a film of antimony on said potassium film,said lm including small amounts of a plurality of alkali metals.

y5. An electron-emissive electrode comprising a supporting base, a ilmof potassium on said base, a lm of antimony on said potassium lm,rsaidlm including small amounts of sodium, potassium and cesium.

6. An electron-emissive electrode comprising a supporting glass base, ailm of potassium on said base, a lm of antimony on said potassium film,said film including small amounts of potassium and lithium.

7. An electron-emissive electrode comprising a supporting glass base, aiilm of potassium on said base, a lm of antimony on said potassium film,said lilm including small amounts of at least two alkali metals.

8. An electron discharge device including an electronemitting portion,said portion comprising a supporting base, a film of potassium on saidbase, a lm of antimony on said potassium lm, said antimony lm includingreaction products of antimony with a plurality of alkali metals.

9. A camera tube comprising an envelope, a face plate at one portion ofsaid envelope, an electron-emissive surface on said face plate andincluding a film of potassium on said face plate, and a lm of antimonyon said film of potassium, said lm of antimony including reactionproducts of antimony with a plurality of alkali metals.

References Cited in the file of this patent UNITED STATES PATENTS2,206,372 Sommer July 2, 1940 2,391,280 Teal Dec. 18, 1945 2,603,757Sheldon July 15, 1952 2,654,048 McGee Sept. 29, 1953 2,747,133 WeimerMay 22, 1956 2,770,561 Sommer Nov. 13, 1956

1. AN ELECTRON-EMISSIVE ELECTRODE COMPRISING A SUPPORTING BASE, A FILMOF POTASSIUM ON SAID BASE, A FILM OF ANTIMONY ON SAID POTASSIUM FILM,SAID ANTIMONY FILM INCLUDING REACTION PRODUCTS OF ANTIMONY WITH APLURALITY OF ALKALI METALS.